This invention relates to tone control circuits employed in stereophonic amplifiers, guitar amplifiers, and so forth.
Heretofore, so-called tone control circuits of the CR type, the NF (or negative feedback) type, or the BAX type are well known in the art as tone control circuits in stereophonic amplifiers. However, the tone control circuit of the CR type has a certain amount of loss in general, and suffers from drawbacks that is necessary to provide an amplifier with low output impedance to drive the tone control circuit and it is difficult to obtain a flat frequency response characteristic. On the other hand, the tone control circuit of the NF type does not require a low impedance driver amplifier. Instead, this type tone control circuit employs a voltage feedback circuit designed to have a desired frequency response characteristic to provide the required tone control. However, the NF type tone control circuit also surfaces from drawbacks. Specifically, its manufacturing cost is relatively high, or it is not economical, because it requires special variable resistors and switches. Since the tone control circuit of the BAX type or the BAX modification type needs two amplifiers, it is disadvantageous in that its circuit is relatively intricate.
Furthermore, known in the art is a tone control circuit disclosed in U.S. Pat. No. 3,683,293 assigned to the same assignee with the present application. This tone control circuit comprises a treble circuit made up of an amplifier circuit with first and second output terminals through which outputs of opposite phases to each other are provided, a first variable resistor connected between the first and second output terminals of the amplifier circuit and having a movable contact grounded, and a series circuit of a resistor and a capacitor connected to the variable resistor. There is further provided a bass circuit made up of a second variable resistor connected between the two output terminals of the aforementioned amplifier circuit and having its movable contact grounded, a capacitor with its one terminal connected to the variable resistor and the other terminal grounded, and resistors connected in series to the variable resistor. In this tone control circuit, the ratio of the load impedance on the first output terminal side to the load impedance on the second output terminal side is varied by varying the resistances of the first and second variable resistors in order to vary the gain in a particular frequency range. However, tone control circuit is disadvantageous in that its manufactuing cost is relatively high.